True he did not age In AGT. But was that not a real timeline. Plus Data did allow himself a streak of grey. Perhaps he can control it. But still that one line was a good out for Spiner to come back.
Irregardless, alternate timeline or not, AGT showed that there is a possible outcome where Data didn’t age, and that that possible outcome could still be reality for him in the actual prime universe. And the streak of gray was voluntarily added by him, it wasn’t a natural process of his programming. Yes, it made him ‘look’ older, but it’s no different from him wearing grandpa pants hiked up to his armpits.